Problem: Simplify the following expression: $y = \dfrac{1}{9t} - \dfrac{2}{5t}$
In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9t$ and $5t$ $\lcm(9t, 5t) = 45t$ $ y = \dfrac{5}{5} \cdot \dfrac{1}{9t} - \dfrac{9}{9} \cdot \dfrac{2}{5t} $ $y = \dfrac{5}{45t} - \dfrac{18}{45t}$ $y = \dfrac{5 -18}{45t}$ $y = \dfrac{-13}{45t}$